The invention is directed to gray and bronze glass compositions having a high transmittance and a high infrared absorption and method of producing the gray and bronze glass.
It would be extremely advantageous to improve the infrared absorption of glass products while maintaining a high level of visible transmission and to also have a good absorption in the ultraviolet portion of the spectrum. Iron oxide exists in two chemical forms in the glass, an oxidized form which is yellow, Fe2O3, and a reduced form which is blue FeO. Advantageously, the oxidized form of iron oxide absorbs a portion of the ultraviolet light passing through the glass product and the reduced form of iron oxide absorbs a portion of the infrared light passing through the glass product. Under typical furnace firing conditions and batching conditions, when the total iron oxide in the glass product is within the range of about 0.2 to 1.2 wt. % as Fe2O3, the iron oxide equilibrium is such that the redox ratio of FeO/total Fe as Fe2O3 is about 0.18-0.26.
It is desirable to increase the proportion of reduced iron oxide (FeO) in the glass to improve its infrared absorption. In addition, by shifting the iron oxide away from the oxidized form (Fe2O3) the glass will change color from green to blue. It would be further desirable to shift the blue glass towards a gray or a bronze color and to simultaneously improve the ultra violet absorption as well as the infrared absorption.
One way commonly employed to shift the redox equilibrium of iron oxide in the glass, and hence its UV and IR properties, is by increasing the fuel to the furnace. Increasing the amount of fuel, however, has several undesirable consequences: the combustion heating of the furnace becomes inefficient and requires an air increase or the unburnt fuel will burn in the checker system of the furnace. Excess fuel can also reduce the glass to an amber color that sharply lowers the visible transmittance of the glass product.
An amber color arises when the iron reacts with sulfur that has been reduced to form iron sulfide. Amber colored glass containers are normally melted in like manner by using anthracite coal together with iron oxide and sulfate. The amber iron sulfide chromophore, once produced, significantly decreases the visible transmittance of the glass and the glass could not be used where a high transmittance is required.
Therefore, there is a need in the glass industry to produce gray or bronze glass that has high transmittance yet having an improved infrared light absorption and an ultra violet absorption.
In one aspect of the present invention a gray and a bronze glass having a base and a colorant is provided. The composition of the base comprises 68 to 75% SiO2, 10 to 18 wt. % Na2O, 5 to 15 wt. % CaO, 0 to 10 wt. % MgO, 0 to 5 wt. % Al2O3, and 0 to 5 wt. % K2O, where CaO+MgO is 6 to 15 wt. % and Na2O+K2O is 10 to 20 wt. % is provided. The composition of the colorants comprises: 0.22 to 0.39 wt. % total iron as Fe2O3 wherein the ratio of FeO/total Fe as Fe2O3 is 0.35 to 0.64; 0.1 to 0.5 wt. % manganese compound as MnO2; 0 to 1.2 wt. % cerium oxide as CeO2; and 2 to 10 ppm selenium, and 0 to 20 ppm cobalt as cobalt oxide.
In yet another aspect of the present invention glass products made according to the embodiment of the invention have the following spectral properties at 4.0 mm. thickness: 65 to 78% light transmittance using Illuminant A (LTA) and using Illuminant C has a dominant wavelength greater than 493 but less than or equal to 577 nanometers with an excitation purity less than 7%. Generally, as the quantities of the colorants increase, both the % LTA and % IR transmittance will go down. Similarly, as the glass thickness increases for a given glass composition, the transmittance of the thicker glass will decrease.
In yet another aspect of the present invention wherein the colored glass of the present invention exhibits an bronze color when the excitation purity is in the range of 2% to 7% and has a dominant wavelength in the range of 560 to 577 nanometers. In yet another aspect of the present invention the colored glass exhibits a gray color when the excitation purity is less than 2% and the dominant wavelength is in the range of 493 to 551 nanometers.